1. Field of the Invention
The invention relates in general to the field of processing materials for subsequent use in semiconductor devices and in particular to a method of forming a narrow gap in such a material.
2. Prior Art
In the fabrication of semiconductor devices, it is often necessary to form narrow gaps in a layer of material. One particular example of this is in the area of charge coupled devices (CCD) in which a silicon substrate is utilized as the charge transfer medium and a series of polycrystalline silicon, often referred to as polysilicon, gates insulated from the substrate and from one another are used both as storage gates and transfer gates. When a four phase clock system is used, two of the gates are used as hold gates and shallow or deep potential wells are formed thereunder by the clock voltages to hold and to assist in the transfer of a "bucket" of charge. The other two gates are placed between the hold gates and act as transfer gates, or valves, to retain and then allow the "bucket" of charge to move from beneath one storage gate to the next upon the application of appropriate clock voltages. In a practical case, the storage gates are 12 microns in width and are separated by a 6 micron gap in which the transfer gates are deposited, each transfer gate overlapping the edge of an adjacent storage gate by 3 microns. The above values are approximately the smallest values obtainable by present high volume manufacturing photolithographic techniques in which there is a .+-.3 micron alignment limit on the placement of a photo resist mask by such techniques, with several masking steps being required to obtain the above gate structure. A single cell under present techniques using a four phase, four gate system, thus occupies a linear dimension of 36 microns and a typical sixteen thousand bit CCD memory would occupy an area of approximately 0.2 cm .times. 0.56 cm. While this dimension may seem small, there is an ever increasing demand to reduce device size and increase device density in order to reduce power requirements and cooling requirements and increase manufacturing yield. While smaller device sizes may be obtained by electron beam techniques, such techniques are quite slow and very costly, thus making them unsuitable for low cost, batch fabricated devices.
Accordingly, it is a general object of the present invention to provide an improved method for forming gaps in materials.
It is another object of the present invention to provide a method for forming very narrow gaps in semiconductor materials using standard masking techniques.
It is yet another object of the present invention to provide an improved method of forming a compact gate structure useful in CCD memories.
It is still another object of the present invention to provide a method of forming very narrow gaps for use with ion implantation techniques.